Improve tooling to enable alerts on performance regression based on data collected from Beam benchmarks

Currently we have some performance benchmarks in Beam, however they are not always monitored and we may not notice when a regression happens, see [1].

Filing this issue to improve the tooling. Note that if automation becomes too noisy (too many regressions) people likely won't be paying attention either.

As a first step, we could monitor regression in 50th percentile of performance metrics we collect for benchmarks that we run repeatedly.

cc: kamilwu kasiak markflyhigh

https://lists.apache.org/thread.html/51e000f16481451c207c00ac5e881aa4a46fa020922eddffd00ad527@%3Cdev.beam.apache.org%3E